1. Field of the Invention
The present invention relates in general to pulverizers for pulverizing coal, and in particular to a new and useful rotating passage segment design for such pulverizers which includes a removable vane design in the welded rotating passage segment.
2. Description of the Related Art
One type of known coal pulverizing mill or pulverizer is a slow speed, roll-and-race-type pulverizer that uses three large-diameter grinding rolls to crush the coal. Primary air enters the pulverizer through a radial inlet duct, moves into a low-velocity air plenum, and is then accelerated and oriented by a series of stationary passages in a ring that surrounds the grinding zone. At the outlet of the passage, the pulverized coal particles are entrained by the high-speed airflow. The velocity of the air is then reduced in the main pulverizer housing causing the larger particles to be returned directly to the grinding zone for further crushing, while the smaller particles are carried up through the classifier for final sizing.
A large portion of the primary air pressure drop is due to losses across the rotating passage. In some known pulverizers, the primary air pressure drop can be about 40% higher than other mills containing different, rotating passage designs. Due to this higher pressure drop, more fan power is required to operate the mill. This results in a large power penalty due only to the passage design.
A known design illustrated in FIGS. 1 and 2, is a modified version of the earlier stationary passage design of the early 1980's. This design comprises a passage arrangement for a pulverizer having a fixed housing 6 with a central axis. Grinding table 16 rotates around the central axis for pulverizing particles, in particular coal, in a conventional fashion. Grinding table 16 rotates in the rotation direction indicated by the arrow in FIG. 2. Air supplied to an inlet plenum 21 travels upwardly through a passage space provided between an outer passage wall 22 and an inner passage wall 24. The passage space is further divided into individual passage ports 10 by vanes 12, distributed in a circumferentially spaced manner around the vertical axis of the pulverizer. This design also consists of 42 passage ports 10 made up of 14 separate castings mounted to the top and bottom of a grinding table 16. The passage is divided into the individual ports 10 through the use of flow vanes 12. The vanes extend from the passage inlet 10a to the passage outlet 10b and are included at an angle 14.degree. of 30.degree. from the horizontal and an angle 18.degree. of 15.degree. from the vertical toward a grinding zone 20. The outer passage wall 22 is stationary while the remainder of the passage including its inner wall 24 and the vanes 12 is rotated with the grinding table 16. The air flow is initially oriented by a tear-drop shape 12a at the leading edge of the vane 12 and is accelerated to promote a uniform velocity profile over an airfoil shape 12b on a portion of the upper surface. Grinding table 16 rotates within a housing 6, about a vertical axis. Wall 22 is supported in the housing and the housing encloses zone 20. The function of the vanes to accelerate and orient the flow through the passage is described in U.S. Pat. No. 4,264,041.
Other pertinent existing prior art relating to pulverizer passage designs are U.S. Pat. No. 2,275,595 (Schwartz '595); U.S. Pat. No. 2,378,681 (Bailey, et al. '681); U.S. Pat. No. 2,473,514 (Ebersole, '514); and U.S. Pat. No. 2,545,254 (Bice '254), U.S. patent application Ser. Nos. 07/858,255 and 07/882,733; all of which are assigned to The Babcock & Wilcox Company. Schwartz '595, discloses curved annular passages fanning a passage discharging scavenging air in the direction of the grinding elements. Bailey, et al. '681, discloses a design for constant air velocity through the passage. Ebersole '541, discloses an adjustable passage, and Bice '254, discloses an eccentric passage design for air distribution. The rotating passage is constructed of 14 individual segments. Each segment includes the vane 12, airfoil 12b, and inner wall 24 being casted as one individual piece. In most cases, only the vanes wear out and need replacement during plant maintenance. Due to the present manufacturing designs of rotating passages, the entire passage segment must be replaced instead of only the worn area. This results in a more time-consuming maintenance period which burdens the customer. The one-piece design of current rotating passage segments result in a more complicated retrofit.